The electron spin resonance (ESR) spectra of the free radicals formed by the autooxidation of serotonin, 5-hydroxyindole, and 5-hydroxytryptophan in 1 N NaOH have been detected. The analysis of the hyperfine splitting constants in H2O and D2O characterize these free radicals as semiquinone-imines, the one-electron oxidation products of the corresponding indoles. At alkaline pH, autoxidation of these compounds ultimately leads to precipitates and unresolved ESR spectra characteristic of polymeric material. The reduction of cytochrome c at pH 7.4 by a wide variety of indoles correlates with the amplitude of the ESR signal in 1 N NaOH, as do other processes thought to be related to 5-hydroxyindole free radical formation. Relative to the rate of cytochrome c reduction, neither serotonin nor the serotonin free radical appears to react with oxygen to form superoxide. In the presence of NAD(P)H, the serotonin radical most probably oxidizes NAD(P)H to form the NAD(P). radical. The NAD(P) radical then reacts with oxygen to form superoxide, which ultimately reduces cytochrome c. This work has been extended to the dihydroxytryptophans, which are neurotoxins.